Mercury-free discharge lamps in which the mercury used in a discharge gas is replaced by other metal halides are known from the prior art. However, if no mercury is provided in the closed bulb, the voltage between the electrodes is reduced such that an increased electric current is required for maintaining the voltage. This results in a higher level of power dissipation from the ballast for the mercury-free discharge lamp in comparison to a conventional mercury-containing discharge lamp. Since, where a lamp with a luminous flux of more than 2000 lm, as is emitted by a conventional mercury-free discharge lamp, is installed, it is mandatory additionally to provide a headlamp windshield washing system and a lamp leveling control, the use of mercury-free lamps as standard equipment was not of interest to automotive manufacturers.
In the prior art, for example in US 2004/0150344, it was therefore proposed that a mercury-free discharge lamp with reduced power requirements and decreased luminous flux be achieved by reducing the dimensions of the discharge bulb and shortening the distance between the electrodes in the discharge bulb. In this way, the temperature in the bulb can, despite the reduced power feed, be kept at a level necessary for a constant voltage.
However, a disadvantage of this lamp known from the prior art is that the light arc which forms in the smaller-sized discharge bulb has too low a spatial extension so that use of these lamps in existing headlamps is not possible.